Toshiba satellite wont boot from a usb

Have you changed the boot order in the BIOS to put USB first in the boot list?

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Good answer and it works(assuming by legacy you mean CSM BOOT) BUT:-
Sorry to burst your bubble but he is still going to have problems as per my Satellite P70-A (PSPLPA) with win8.1.
At this point please understand that a Boot Windows 10 Install USB handles both BIOS and UEFI correctly.
The problems with the suggested solution are:-
1. You will boot to USB ok BUT only in BIOS mode (uses bootmgr) NOT UEFI mode (uses bootmgfw.efi)
Of course you can try deleting bootmgr from the install disk to force a UEFI boot(as some scripts have suggested) but it still errors.
2. If you are installing another Version of Windows (such as win10) then It will get to the windows setup screen and ask you the question,
"What partition do you want to install into?" Now only CPT partitions are showing, and Windows will tell you it cant install into any of them.(they arnt MBR format)
Now the crucial part here is do you want to create an obsolete PC that runs?, do part A. Or do you want UEFI (the replacement for BIOS) as your operating system, then see Part B.
Part A. Because your windows install has booted on USB as BIOS it wants MBR partitions on the disk (formated with NTSF) so you can get this by deleting every partition shown (created in win 8 as CPT needed for UEFI) and select the unallocated space and hit NEXT to install a backdated version with BIOS and forever leaving your PC in Legacy mode.
PartB. Personally I want UEFI (which is designed to replace BIOS) which formats unlimited partitions instead of max 4 as does MBR , works faster at boot and shutdown, and does not have a max of 4TB per partition. Lets face it I bought a new PC to get all that right! OK so it is all based on what WIN 10 is booted with on the USB. If UEFI worked, (which as per this forum it dosent) and Win10 booted as UEFI it appears that we can still delete every partition shown us on the windows setup screen, (possibly old MBR partitions, as well as old CPT partitions put there by window 8.1,) and select the remaining unallocated space and click NEXT. Win 10 will recognise itself running as UEFI and continue by formatting all partitions it needs as CPT, thereby giving us the latest UEFI technology PC.
My corollary is this: Toshiba machines wont allow Windows 10 to boot properly so it can choose what firmware to sync with, therefore Windows 10 wont install correctly on UEFI PC's.

OK I confess it is a wrong corollary. No one told me that to run UEFI you have to format it first.
So To bypass the error booting to USB either
1. Do Tims answer to create a BIOS USB or
2. Do this to create a UEFI USB:-
You have to preformat the USB drive as EFI first then load the .iso
preformatting was found at
http://www.eightforums.com/tutorials/15458-uefi-bootable-usb-flash-drive-create-windows.html
Once this was done using rufus, the Toshiba with secure boot on, and boot mode as UEFI, booted from the efi USB ok.
(Put USB first in boot menu order)
rufus is found at: http://rufus.akeo.ie/
So Problem solved.

ROYC34's most recent post actually included the answer to this riddle, but just to explain things differently from someone else(me) that just went through this headache....I struggled with this for quite some time myself.
I'm using an L15W-B1302 with the latest BIOS rev available(v1.20) that does NOT have the ability to toggle to CSM mode. Here is what worked for me:
1) use Rufus(https://rufus.akeo.ie/) to build the bootable USB, not Microsoft's "Windows USB/DVD Download Tool".
2) Connect the destination flash drive that is to be written to prior to launching the Rufus app.
3) before setting partition scheme, file system, or cluster size, browse to and select the ISO image file to be used using the "Click to select an image..." button which is to the right of the "Create a bootable disk using" checkbox.
4) Now, set the "Partition scheme and target system type" to "GPT partition scheme for UEFI computer".

Notes: I used FAT32 as the File system option and 4096 bytes as the cluster size and my CSM-optionless Satellite was able to boot off of and successfully install Windows 10 Pro Insider Preview -10074 x64. I haven't tried using NTFS or 8192 byte cluster size, but they may possibly work as well.

The crux of the problem, I believe, stemmed from the fact that I could select "GPT partition...", but when I then pointed Rufus at the ISO, it "reset" the "Partition scheme..." setting back to "MBR..." without asking or notifying me. Make sure your partition sceme is set to "GPT..." when you click "Start" on writing the ISO to the USB flash drive and you should be fine.